The present invention is directed toward a method for performing a seismic survey and processing the data received from said survey, more particularly, a method for activating a seismic source and processing the survey data in a manner that effectively focuses seismic signal return on a selected point within a subterranean formation.
A seismic source is a device that releases energy in the form of an acoustic wave. Basically there are two types of seismic sources, an impulsive seismic source of which the waveform cannot be controlled and a waveform-controllable seismic source. Examples of an impulsive seismic source are an explosive device, a dropping mass or an implosive device, and the signal generated by an impulsive seismic source is a pulse. The waveform of an impulsive seismic source cannot be controlled. A waveform-controllable seismic source is a vibrator that generates a controlled wave train. The seismic sources are used to make a seismic survey of the structure of an underground formation.
Making a seismic survey of the structure of an underground formation comprises activating the seismic sources to generate seismic waves; allowing the seismic waves to travel through the underground formation in which they are reflected by acoustic-impedance contrasts and refracted through members having a variable velocity; and receiving and recording the seismic waves by seismic receivers. The arrival times of the seismic waves are observed to obtain a result that can be interpreted. Common seismic survey techniques generally include one or more seismic sources and multiple seismic receivers, and generating and recording multiple sets of seismic waves. Source and receiver placement may vary, with both source(s) and receivers being located on the ground, in the formation of interest or some combination of both. Similarly, in a marine environment, the sources and receivers may be towed, on the sea floor or a combination of both. In a borehole seismic survey, receivers, sources, or both may be placed in a well, at various formation depths to perform.
It is an object of the present invention to use the information obtained from the seismic survey to determine how to activate a seismic source such that the seismic energy receive in a target point in the undergound formation has a prescribed shape.
As observed above, there are two kinds of seismic sources, impulsive seismic sources and waveform-controllable seismic sources. For the impulsive seismic sources it is an object to concentrate the seismic energy in a target point.
In the present method of concentrating seismic energy, a borehole seismic survey is made with multiple impulsive seismic sources i on the surface and multiple seismic receivers i placed in multiple boreholes at the formation depth of interest. A seismic survey is made in which the sources i are activated and the signals are recorded by the seismic receivers to form the data set uij(t). target position k in the formation at depth of interest is then selected as the focus point. The one-way travel times from each sources to the target point xcfx84ik is then calculated from the received signal set uij(t). An activation time ti=t0xe2x88x92xcfx84ik is calculated for each source, where t0 is the time at which the energy from all of the sources arrives at target k in phase. The seismic array sources are then activated according to ti to provide a focused source wherein all the energy arrives at target k at t0 and a second received signal set is created.
This method is suitably applied in case the target point coincides with a seismic receiver k. However, in case the target point l does not coincide with a seismic receiver k, a correction has to be applied. The correction on the activation time of the sources i is xcex94ti=xcfx84ikxe2x88x92xcfx84il, wherein xcfx84il is the one-way travel time from source i to the target point l. The corrected activation time for the seismic source i is then tcorri=ti+xcex94ti.
Suitably the one-way travel times are calculated with a velocity model that is obtained from signals uim(t), wherein m refers to a set of seismic receivers, at least one of which is located at surface.
For the waveform-controllable seismic sources it is an object of the present invention to enable concentrating seismic energy in a selected target point in an underground formation, wherein the wave at the target point has a predetermined desired waveform.
In this instance, at least one focused waveform-controllable seismic source i is located on the surface and a set of downhole receivers i located in a subterranean formation. A seismic survey is made wherein said at least one seismic source i is activated the waveform(s) are recorded by each receiver i to form a data set uij(t), where each source is activated by a signal si(t). A target k is then selected in the subterranean formation from the set of seismic receivers to be the target point k. The direct part udik(t) of the signal uik(t) is obtained and used to create an inverse filter (udik(t))xe2x88x921. A waveform is then selected fk(txe2x88x92t0) to be focused at the downhole seismic receiver k at time t=t0. Based on the above, a new activation sequence for seismic sources i is determined snewi(t)=si(t) (udik(t))xe2x88x921 fk(txe2x88x92t0) resulting in a focused source I in which the waves arrive at the downhole receiver k with the desired waveform fk(t) a time t0.
As with the impulsive seismic source, the present invention further contemplates a target l that does not coincide with receiver k. A correction factor is determined by xcex94tcorri=xcfx84ikxe2x88x92xcfx84il where xcfx84il is the estimated one-way travel time from source I to target point l, such that snewi(t)=snewi(t+xcex94ti).
Suitably, the one-way travel time to the target location is calculated using a velocity model obtained from signals uim(t), wherein m refers to a set of seismic receivers, at least one of which is located at surface.
In the specification and the claims the symbol  denotes convolution.